[CI] BuildNet10_0App fails to link _mono_helper_box_nullable #26474

Failure signature

BuildNet10_0App fails while linking iOS and Mac Catalyst applications because the AOT-generated System.Private.CoreLib.dll.o references a native runtime symbol that is unavailable:

Undefined symbols for architecture arm64/x86_64:
  "_mono_helper_box_nullable", referenced from:
      wrapper_managed_to_native_object___icall_wrapper_mono_helper_box_nullable_intptr_intptr
      in System.Private.CoreLib.dll.o
ld: symbol(s) not found
clang++: error: linker command failed with exit code 1

Confirmed occurrences

Total: 8 confirmed test occurrences across 2 distinct PRs, 4 heads, and 4 Azure runs.

First observed: 2026-08-25. Latest observed: 2026-08-27.

Classification and impact

Deterministic source/runtime linkage failure. The same symbol, generated wrapper, and linker failure reproduce on iOS arm64 and Mac Catalyst x86_64 across multiple dependency-update heads. This blocks the .NET 10 compatibility test in both platform jobs.

Run 15114195 uses .NET SDK 11.0.100-rc.2.26426.102 with the 26.5.10315 .NET 10 Apple SDK packs. The iOS failure ran on AcesShared 20; the Mac Catalyst failure ran on AcesShared 21.

No retry was requested because unchanged retries cannot repair a deterministic runtime/native-pack mismatch. Later #26454 heads stopped reproducing this signature, but it has recurred on #26356; the exact fixing commit was not established from retained diagnostics.

Investigation performed

• Inspected Azure task logs and NUnit/HTML artifacts for all current failures.
• Confirmed the exact _mono_helper_box_nullable symbol and System.Private.CoreLib.dll.o reference on both architectures.
• Searched all open and closed dotnet/macios issues by stable signature, exact symbol, test name, linker error, and component. No issue represents this exact root cause.
• [CI] DotNetWatch CoreCLR Hot Reload crashes with Bad IL range #26470 tracks a different post-Hot-Reload Bad IL range failure and only mentions this symbol as a separate, previously resolved validation outcome.
• Older undefined-symbol issues involve different native symbols and causes.

Reproduction pattern

On an affected dependency-update head, build the .NET 10 simple application through:

• Xamarin.Tests.DotNetProjectTest.BuildNet10_0App(iOS, "ios-arm64")
• Xamarin.Tests.DotNetProjectTest.BuildNet10_0App(MacCatalyst, "maccatalyst-x64")

The managed AOT object references _mono_helper_box_nullable, but the linked native runtime libraries do not provide it.

Workaround

No confirmed workaround is available. Aligning the managed runtime/AOT output and native runtime pack resolves the underlying class of mismatch; the exact package or commit change still needs to be identified.

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